In several processes for manufacturing chemicals or for processing materials gas streams are produced that are subject to entrainment of droplets of liquid carried over from prior processing, for example droplets of water or liquid hydrocarbons. Entrained droplets of small size are referred to as mist. The streams containing entrained droplets are termed wet gas. Frequently, it is desirable to remove entrained liquid as it will adversely affect further processing of the gas stream. In these cases it necessary to remove the entrained droplets, one method for which is to use a wet gas separator. Such a process also may be termed de-entraining, entrainment separation or demisting.
Several designs are known for separation of entrained droplets of liquid from gas streams. A wet gas separator typically uses a contact system to cause the entrained droplets to accumulate into a stream of liquid that is separate from the gas stream. Frequently, the liquid droplets are treated as particulate matter, and removed using filters, cyclones and other means, as outlined for example by de Nevers in “Air Pollution Control Engineering” 2nd Edition, McGraw Hill (2000), pages 414-415. When a cyclone is used, the liquid droplets are driven centripetally to accumulate at the outer wall of the cyclone and coalesce. A simple system for effecting contact between droplets and coalescence is a wire mesh mist eliminator, for example for collection of sulfuric acid mists, performance of which is in Section 4.4.4 of “Handbook of Environmental Control—Volume 1: Air Pollution” Edited by Bond et al., CRC Press. (1972). The liquid droplets of the mist contact the mesh and combine there to form an extended liquid mass which then falls under gravity toward a drain for removal.
Mesh-based demisters can be vertically or horizontally oriented. When the apparatus is vertical the mesh is horizontal or inclined, and the accumulated liquid flows downward to a bottom drain, as described by, for example, Carns et al. in U.S. Pat. No. 6,964,699 (2005). Some separators have a plurality of components to effect good liquid-gas separation, as described by Van Egmond et al. in U.S. Pat. No. 7,074,979 (2006), and may rely at least in part with partial flooding of the apparatus so that the droplets contact bulk liquid as described by, for example, Van Egmond in '979 and by Ross et al. in U.S. Pat. No. 5,972,171 (1999). Force of flow may be used to cause droplets to contact contiguous surfaces while gas passes relatively unhindered through successive changes in direction in a de-entrainment chimney, and such impingement effects coalescence of the droplets, as described by, for example, Chosnek et al. in U.S. Pat. No. 6,599,348 (2003), Silvey in U.S. Pat. No. 4,698,138 (1987), Caesar in U.S. Pat. No. 4,316,728 (1982).
Baffles, too, may be situated at different angles to effect droplet separations, as described for recovery of oil from oil-gas mixtures by Miller in U.S. Pat. No. 6,048,376 (2000).
Centripetal forces are used to cause droplet-surface contacts in a centripetal demister described by Miles in U.S. Pat. No. 6,451,093 (2002).
Cyclone systems may operate with the rotation of gas conventionally about a vertical axis or about a horizontal axis, as described by Suh et al. in U.S. Pat. No. 4,617,031 (1986). A combination of rapid depressurizing and cyclonic action is used in a demisting chamber with elbow strainer described by Wydra et al. in U.S. Pat. No. 7,306,639 (2007). A combination of cyclonic action and wall contact arising from a sudden change in gas flow direction is described by Zarif in U.S. Pat. No. 6,691,428 (2004).
Different types of separator may be combined within one apparatus, as described by Huber et al. in U.S. Pat. No. 7,025,808 (2006) and Savage et al. in U.S. Pat. No. 6,045,660 (2000).
Wet electrostatic precipitators also may be used to remove droplets as described by, for example, Paranjpe et al. in U.S. Pat. No. 6,106,592 (2000).
It has been found that, for some processes under a variety of conditions, present equipment is subject to breakthrough of liquids into the effluent gas stream. The breakthrough occurring with some designs may be caused by re-entrainment of the collected liquid. Thus it is advantageous to capture re-entrained liquids. What is needed is a wet gas separator having better capability for separation of entrained liquid and re-entrained droplets, thereby preventing liquid breakthrough. The present invention achieves this goal through an improved design having one or more of a vaned inlet by which flow patterns within the separator are amended, an droplet removal system having improved capability to de-entrain droplets, and an exit system with which the exhaust stream can be fined.